In recent years, the operating frequencies of various electronic devices have increased, and the package sizes, especially the heights, have decreased. According to such an increase in operating frequency and a reduction in package size, there is also a need for piezoelectric resonator devices (crystal resonators, for example) to be adaptable to the increase in operating frequency and the reduction in package size.
In this kind of piezoelectric resonator devices, a housing is constituted by a rectangular-shaped package. The package is constituted by: a first sealing member and a second sealing member both made of glass or crystal; and a crystal resonator plate made of crystal. On both main surfaces of the crystal resonator plate, excitation electrodes are formed. The first sealing member and the second sealing member are laminated and bonded via the crystal resonator plate. Thus, the excitation electrodes of the crystal resonator plate that is disposed in the package (in the internal space of the package) are hermetically sealed (e.g. Patent Document 1). Hereinafter, such a laminated structure of the piezoelectric resonator device is referred to as a sandwich structure.